Field of the Invention
The present invention concerns a method and a magnetic resonance system to detect MR data of a predetermined volume portion of an object under examination, particularly in the case of magnetic field inhomogeneities.
Description of the Prior Art
A SEMAC method can be used to suppress metal artifacts in SE (spin echo)-based magnetic resonance data acquisition sequences (for example TSE (turbo spin echo)), see “SEMAC: Slice Encoding for Metal Artifact Correction in MRI”, W. Lu et al. Magnetic Resonance In Medicine 62, pages 66-76, 2009. In this context, with a conventional two-dimensional protocol or slice-by-slice excitation, additional encoding is performed in the slice direction. This encoding corresponds to phase encoding and is also known as so-called SEMAC encoding. In this context, the measuring time increases linearly with the number of additional phase encoding steps in the slice-selection direction, which are also known as SEMAC steps. In particular in the case of T2-weighted TSE protocols with a long repetition time TR, this increases this entire measuring time markedly.
For example, a measuring duration of 2 minutes and 8 seconds is required to detect a slice with 256 phase encoding steps with a TSE sequence with a turbo factor of 8 and repetition time TR of 4 seconds. If eight SEMAC steps are used to suppress metal artifacts, the measuring time increases to more than 17 minutes, which is clearly too long for clinical protocols.
This long measuring time places significant stress on the gradient system of the magnetic resonance system, which can disadvantageously result in a limit defined, for example, by the duty cycle (for example defined by a percentage with which the first phase encoding gradient is switched over time) being exceeded.